Question migration from an existing forum into a Stack Exchange database is possible, however there are significant problems. In fact, Stack Overflow was originally intended to have been seeded from the Joel forums, thus post #1, "Where, oh where, did the Joel Data go?" (deleted, 10K users only).
It was determined at that time that forum questions look and work horribly under the new format.
It's even more true now that we have comments. We can assume the first post was the question, but even with good threading it's hard to figure out (except by hand) which of the following posts are actually replies to other posts, or to the original question.
It's a mess and at the time they determined that it wasn't really going to add much to Stack Overflow, and this is without even considering trying to get the database from the forum owner, and figuring out what extensions they've installed and how to deal with extraneous data, dealing with the multiple types of forum software, nevermind the multiple versions of each type.
Now we encourage people to re-post such questions to Stack Overflow, and let the answers come from existing users, although one could go to the trouble of moving the important, relevant information over by hand for answers and comments as well, it would be confusing since the user database couldn't and wouldn't match.
In short, it's too much work, for too little benefit. Forum based question and answer groups might as well be inaccessible data silos - the mismatch is simply too great.
Forums that are interested in integrating Stack Overflow into their forum have many avenues. An iframe which shows the search page based on the open forum thread might be useful for some. The Stack Overflow API can be used to pull specific questions and results from the database if you want to display them in a custom format. The data dump is available for those that want to do a lot more with the entire database, though two months behind, and there are numerous RSS feeds one can pull from.